The importance of ceramic actuators as solid displacement elements has increased in recent years with the development of miniaturized, intelligent electronic equipment. Today piezoelectric ceramics and electrostrictive ceramics are known as ceramic actuator materials, and with respect to the piezoelectric ceramics, they have already been used in many fields. On the other hand, although electrostrictive ceramics do have preferable actuator properties than piezoelectric ceramics, it is difficult to synthesize pure compounds by conventional ceramic synthesis methods and the amount of displacement of electrostrictive ceramics that are obtained by conventional method is approximately half compared with that of piezoelectric ceramics. Therefore, practical use of electrostrictive ceramics for displacement elements has been delayed. Since electrostrictive ceramic actuators have advantages in that their hysteresis and change over time in strain are less than that of piezoelectric ceramic actuators, their use is expected in fields where high accuracy and precise control of the amount of displacement are required (reference: "From Piezoelectric/Electrostrictive Actuator, Fundamentals to their Actual Use": Kenji Uchino, Morihoku Shuppan, 1994).
Today a solid solution of the composite perovskite-type compound Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 (referred to as PMN below) and the primitive perovskite-type compound PbTiO.sub.3 (referred to below as PT), that is a so-called PMN-PT solid solution, is being developed as an electrostrictive ceramics for practical use. This compound shows an extremely high relative dielectric constant at room temperature in the region of a composition of approximately 0.9 PMN-0.1 PT, and generates relatively large electric-field induced strain (.about.1 .mu.m/mm). However, this amount of strain is about half compared with that of (Pb, La)(Zr, Ti)O.sub.3 solid solution (PLZT), which, of the piezoelectric ceramics, is known to generate particularly large electric-field induced strain, and is not sufficient for an actuator material. Furthermore, the composition of a PMN-PT solid solution with which extreme strain is. obtained has a high ratio of Nb.sub.2 O.sub.5, which is expensive, and it is difficult to obtain single-phase perovskite by ordinary ceramic synthesis methods. Thus, there is a problem with the current electrostrictive ceramics in that in addition to the fact that the amount of displacement that is obtained is small, the degree of freedom of material design is very limited.
Therefore, the inventors performed intense studies of electrostrictive materials showing large displacement in light of the above-mentioned prior art and as a result, they completed the present invention upon discovering that Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 solid solution, which is known to show the same very high relative dielectric constant as PMN-PT solid solution, has an excellent electrostrictive property and is useful as electrostrictive material. This solid solution is known to have a very high relative dielectric constant near room temperature when 30 molar % primitive perovskite-type compound PbTiO.sub.3 is made into a solid solution with the composite perovskite-type compound Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 (referred to below as PNN). However, it is difficult to obtain single-phase particles of perovskite in a PMN-PT solid solution by conventional ceramic synthesis methods and therefore, there has been very little research of its properties. Nevertheless, as a result of showing that particles of high purity can be easily obtained by the synthesis methods developed by the present inventors in recent years, they have discovered that this material has excellent electrostrictive properties and is useful as electrostrictive ceramic.